VULCAN SUB-BASIN FAULT STYLES — IMPLICATIONS FOR HYDROCARBON MIGRATION AND ENTRAPMENT

Abstract

Several structural domains are recognised within the Vulcan Sub-basin, Timor Sea. These domains developed during the Jurassic rifting phase and are separated by major transfer zones which trend in a northwest-southeast direction. Within each domain are frequent third order transfers which sub-divide the main northeast trending fault blocks into numerous compartments. These enable structural hydrocarbon traps to be formed, despite a predominant regional dip. They also affect migration pathways.

Jurassic fault styles include detached rotational blocks, salt-associated features, tilted fault blocks and 'hourglass' horsts and grabens. These generally have a northeast-southwest orientation. The transfer faulting complicates these features and forms zones of structural complexity with associated poor seismic data quality. A separate fault episode in the north of the sub-basin during the Tithonian resulted in an east-west fault set overprinting the earlier structuring.

Intra-Cretaceous fault movement is also recognised and has an important role in early hydrocarbon entrapment.

Structural reactivation during the Late Miocene/Early Pliocene of the earlier fault sets modified the geometry of many existing traps. Numerous new traps may also have formed as a result of this tectonism. In many places the resulting geometry is complex, particularly where the younger fault orientation is at an angle to the main Oxfordian fault set. The late-stage movement is primarily extensional, manifested by predominantly normal faults; overall, however, a varying component of strike slip is likely. A divergent strike-slip zone is recognised at the southwest end of the Cartier Trough.

The effects of the late stage tectonism tend to mask the seismic expression of Mesozoic hydrocarbon traps resulting in many wells being drilled off-structure at the target horizon. An understanding of the deeper structuring should result in further discoveries in this prospective basin.